C-terminal sequence analysis of proteins provides necessary information for characterizing intact proteins. The information may be used to determine the presence of postranslation events at the C-terminus, including proteolytic processing. This information, together with N-terminal sequence data, may also be used to clone proteins by a PCR approach. Previously, we developed a chemical method of C-terminal sequencing that allows analysis off 1-5 amino acids at the 0.02 - 1.0 nmole range. In this application we expand the utility of our method by improving repetitive yields through a careful analysis by mass spectrometry at each step of the process on model peptides and proteins. New reagents, based on the original reagent diphenylphosphoroisothiocyanatidate, will be synthesized and tested for their effectiveness in peptide bond cleavage at the C- terminus and at internal aspartyl residues.